1. Field of the Invention
This invention relates to apparatus and methods for measuring the heating value of gaseous fuels and, more particularly, to apparatus and methods for measuring the heating value of gaseous fuels having low heating values.
2. History of the Prior Art
The calorific value of a combustible gas has been defined as the quantity of heat in British Thermal Units (BTU) which is released when one standard cubic foot of gas is completely oxidized at a temperature of 60.degree. F. and any water produced by oxidation is in the liquid state. When the gas is a hydrocarbon or a mixture of hydrocarbons, the oxidation products of complete oxidation are carbon dioxide and water; and when one standard cubic foot of the gas is mixed with a sufficient quantity of oxygen at 60.degree. F. to completely oxidize the gas, oxidation is carried out and the products thereof, carbon dioxide and water, are cooled to 60.degree. F. and all water is condensed to liquid state. The total heat given off, including the heat transferred in cooling the products and in condensing all of the water, is the calorific value of the gas.
Calorific value so defined is used extensively in industry as a measure of the quality of a gas or other fuel. If a gas is to be fed to a burner, the proper operation of the latter is often highly dependent upon calorific value whereby it is essential to control such value within narrow limits. Accordingly, it is common practice for suppliers and users of combustible gas to monitor the calorific value thereof.
Since the calorific value of a combustible gas depends only on its chemical composition, that value can be determined by a complete chemical analysis of the gas if the calorific value of each of its constituents is known. However, this method is time-consuming and impractical for continuously monitoring the calorific value.
Many standard methods for measuring calorific value involve mixing and burning a known volume of a combustible gas with an excess of oxygen-containing or combustion-supporting gas, transferring the resultant heat to a heat absorbing fluid and measuring the quantity of heat transferred. The conditions for these operations would ideally be the same as in the above definition of calorific value. Any deviation from these conditions will cause the resultant heat transferred per standard cubic foot of combustible gas in the measurement to be different from the calorific value.
In practice, it is difficult to maintain the rigid conditions required for correct measurement. Part of this difficulty stems from the initial temperatures of the combustible and oxygen-containing gases seldom, if ever, being 60.degree. F. Also, the temperature of the combustion products after heat transfer is not 60.degree. F. and, usually, is higher than the initial temperature of the gases. The water produced is seldom condensed to the liquid state and the heat absorbing fluid never absorbs all of the heat transferred from the combustion products, since some heat is always lost by radiation and conduction.
Each of the foregoing deviations is a source of measurement error and correction thereof requires complicated, expensive apparatus and, often, special environmental control.
In response to the above, methods of and means for measuring the calorific value of combustible gases which do not depend upon measuring the amount of heat released in combustion, which are not affected by the errors discussed above, which are capable of continuous operation, and which are not affected by ambient temperature and other varying environmental factors have been developed. One such method and means is disclosed in U.S. Pat. No. 3,777,562 to William H. Clingman, Jr. Briefly, Clingman teaches burning a mixture of a combustible gas and a combustion supporting gas in one or more flames, monitoring the temperature or temperatures of the burned gases, and adjusting the volume ratio of the combustion-supporting gas to the combustible gas so as to maintain the temperature or the average of the temperatures at substantially maximum (i.e., at the stoichiometric point). Because, as taught by Clingman, the volume ratio of the gases which produces the maximum temperature (i.e., the volume ratio at the stoichiometric point) varies substantially directly with the calorific value of the combustible gas, the calorific value of the combustible gas may be determined.
"Stoichiometric combustion" instruments, such as that invented by Clingman and described above, are both accurate and rapid in operation when they can be employed. However, they can only be employed when the gas being analyzed will form a combustible mixture with air. In certain situations, such as those involving flares, it is often necessary to measure gases with low heating values, e.g., 100-200 BTU/scF. These gases may not burn in a premixed flame (although they may burn in a diffusion fed flame, and are in that sense combustible); thus, the heating value of these gases may not be measured by the "stoichiometric combustion" (e.g., Clingman) apparatus, instruments, and methods described above. Additionally, the "stoichiometric combustion" apparatus, instruments, and methods described above always have limited ranges. This is because there is a limited air-fuel ratio for which a flame will be stable in a given burner.